1. Field of the Invention
The invention relates to an electrical actuator comprising a magnetically permeable stator body and a permanently magnetic rotor body which is pivotable relative to the stator body about an axis of rotation through a limited angle of rotation, the stator body comprising electrical energizing means for exerting an electromagnetic torque on the rotor body, said electromagnetic torque being substantially proportional to a value of an electrical current through the energizing means and being dependent on an angle of rotation of the rotor body relative to the stator body in accordance with a first function of the angle of rotation, the stator body further exerting a magnetostatic torque on the rotor body which is dependent on the angle of rotation in accordance with a second function of the angle of rotation.
The invention also relates to a throttle device for use in an air inlet of an internal-combustion engine, which throttle device comprises a throttle-valve housing, an air passage which is connectable to the air inlet, a throttle valve which is journaled in the throttle-valve housing so as to be pivotable in the air passage, and an electrical actuator for pivoting the throttle valve.
2. Description of the Related Art
An electrical actuator and a throttle device of the kinds mentioned in the opening paragraph are known from WO-A-95/34903. The throttle valve of the known throttle device is pivotable in the air passage by means of the known actuator. The stator body of the known actuator is provided with permanent auxiliary magnets which exert an additional magnetostatic torque on the rotor body. Under the influence of said additional magnetostatic torque, the rotor body of the known actuator and the throttle valve coupled to the rotor body are urged into a rest position when no electrical current is supplied to the energizing means. When a current is supplied to the energizing means, the rotor body and the throttle valve are pivoted through a limited angle of rotation about the axis of rotation into an operational position under the influence of the electromagnetic torque exerted by the energizing means. The known actuator is provided with an electronic control unit for controlling the current through the energizing means. The control unit is provided with a feedback control loop comprising a sensor for measuring an actual value of the angle of rotation of the rotor body relative to the stator body. The control unit further comprises an electrical input for receiving an input signal which corresponds to a desired value of the angle of rotation. The control unit controls the current through the energizing means such that the actual value of the angle of rotation equals the desired value of the angle of rotation.
A disadvantage of the known actuator is that the operational positions into which the rotor body can be pivoted under the influence of the electromagnetic torque have a very low stability. As a result, the known actuator must be provided with a relatively complicated control unit, in particular a digital control unit, to avoid disturbances of the desired angle of rotation of the rotor body which may arise under the influence of, for example, external loads exerted on the throttle valve.